ip xfrm considers that the user-defined mark is "any" as soon as
(mark.v & mark.m == 0), which prevents from specifying non-wildcard
marks that include the value 0 (typically 0/0xffffffff).

Yet, matching exactly mark 0 is useful for instance to separate
vti policies from global policies.

Always configure the user mark if mark.m != 0.
Signed-off-by: Christophe Gouault <christophe.gouault@6wind.com>